Removed most of mirrors.js and debug.js.
Further steps:
- migrate liveedit.js to native,
- remove debugger context.

R=yangguo@chromium.org
TBR=leszeks@chromium.org

Bug: v8:5530
Cq-Include-Trybots: luci.chromium.try:linux_chromium_headless_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I291ef20ef3c63a424d32e3e0c9d0962a6ca382d1
Reviewed-on: https://chromium-review.googlesource.com/1081176
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53480}